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- Author or Editor: W. A. Oitto x
Pear cultivars, previously rated as moderately to highly resistant to fire blight, were subjected to further natural infection, supplemented by artificial inoculation with Erwinia amylovora (Burr.) Winsl. et al. in various tissues. Of 49 cultivars evaluated, 69% remained in the same class or one class below their previous rating. There appeared to be very little correlation between the amount of blossom blight and overall degree of blight resistance in cultivars. Pear cultivars were shown to exhibit varying degrees of blight resistance in succulent shoots and woody trunk tissue.
Natural infections of fire blight following 2 years of epiphytotic conditions caused severe blight damage in the pear cultivar collection at Beltsville, Maryland. Of 522 cultivars rated with the USDA fire blight scoring system, 88% were highly susceptible, 2% were moderately susceptible, 4% were moderately resistant, 5% were highly resistant, and 2% had no blight symptoms. Names of pear cultivars in each resistance class are listed.
Of 20 cultivars of pear (Pyrus spp.) evaluated for their degree of resistance to fire blight, caused by Erwinia amylovora (Burr.) Windslow et al., following 6 years of epiphytotic conditions, 9 were rated as resistant: (in descending order) ‘Magness’, ‘Moonglow’, ‘El Dorado’, ‘Cornice’, ‘Maxine’, ‘Mac’, ‘Dawn’, ‘Duchess d’Angouleme’, and ‘Kieffer’. Small plots of trees, sufficient to determine very susceptible cultivars, are not adequate to evaluate resistant cultivars.
We rated 86 pear cultivars and selections for their grit content on an inverted scale of 9 to 1. Of these, 64% were rated 6, which we consider to be commercially acceptable, or higher. More than 2000 seedlings from 186 crosses with these cultivars and selections as parents were grouped on the basis of their parental mean grit rating. In each case where the parental mean was 5.0 or higher, the progeny mean was lower than the parental mean. Conversely, in each case where the parental mean was 4.5 or lower, the progeny mean was higher than the parental mean. Where the parental mean was 7.5 or higher, 84% of the seedlings had grit ratings of 6 or higher. Conversely, where the parental mean was 4.0 or lower, only 26% of the seedlings had grit ratings of 6 or higher. We can thus predict the percentage of seedlings in a progeny that can be expected to have a commercially acceptable grit rating. Grit cell development appears to be inherited quantitatively at a minimum of 4 loci. The genes appear to be additive in action rather than dominant.